A new type of continuous casting mold powder for austenitic stainless steel and application thereof

Abstract

The application belongs to the technical field of continuous casting protective slag, and discloses a novel continuous casting mould protective slag for austenitic stainless steel and application thereof. The percentage content of each component of the protective slag is as follows: wollastonite 21.00-29.00%, lepidolite 18.50-21.50%, fluorite 15.00-17.00%, limestone 13.00-17.00%, sodium carbonate 9.00-11.00%, glass slag 4.50-5.50%, borax pentahydrate 3.75-4.25%, magnesia 1.75-2.25%, manganese carbonate 1.00-2.00%, graphite 1.00-1.50, and carbon black 1.00-1.50. In the application, the use of appropriate amount of lepidolite under the synergistic effect of other components not only significantly reduces the production cost of the steel enterprise, but also solves the problem of slag entrapment caused by the denaturation of Cr2O3 in the austenitic stainless steel in the continuous casting process due to the absorption of the protective slag, and improves the surface quality of the white plate product after the stainless steel is washed.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of continuous casting protective slag, and particularly relates to a novel continuous casting mold protective slag for austenitic stainless steel and application. BACKGROUND

[0002] In the continuous casting process of the steel industry, especially to improve the continuous casting speed for adapting to the fast-paced production nowadays, the temperature of the crystallizer liquid steel is also increased to a certain extent, and the continuous casting protective slag becomes one of the key technologies for the development of continuous casting. The continuous casting protective slag has five metallurgical functions, including heat preservation, absorption of non-metallic inclusions in the liquid steel, prevention of secondary oxidation of the liquid steel by air, lubrication effect in the crystallizer and initial solidification shell, and uniform heat transfer from the shell to the crystallizer wall through the formed slag film. When the continuous casting production speed is increased, the melting point of the continuous casting protective slag needs to be reduced and the melting speed needs to be accelerated, so that a sufficient thickness of liquid slag can be filled in the inner wall of the crystallizer and the initial solidification shell to provide sufficient lubrication performance. However, the melting speed of the continuous casting protective slag is not the faster the better, and too fast melting speed will reduce the heat preservation performance of the protective slag; if the melting speed is too slow, the liquid slag supply will not be enough, which will also induce the sticking and leakage accident.

[0003] The Cr2O3 formed by the oxidation of about 18% Cr element in the austenitic stainless steel under high temperature conditions will be absorbed by the molten liquid protective slag, thereby deteriorating the performance of the protective slag. Specifically, with the increase of the Cr2O3 content in the liquid protective slag, the melting temperature and viscosity of the liquid protective slag are significantly increased, and the total heat transfer rate is decreased with the increase of the Cr2O3 content, which is due to the decrease of the liquid phase layer and the decrease of the radiation heat transfer. At the same time, according to the single and double wire thermocouple test results, the increase of Cr2O3 will also inhibit the crystallization of the protective slag, thereby reducing the heat control efficiency of the slag film, which is easy to cause the surface cracking of the casting shell and produce slag penetration. The element content of a certain 300 series austenitic antibacterial stainless steel is as follows: C: 0.025-0.065%; Si: 0.30-0.70%; Mn: 0.50-2.00%; P: ≤0.05%; S: ≤0.006%; Cr: 17.50-19.00; Ni: 7.50-8.50; Mo: ≤0.60%; Cu: 0.50-1.00; N: 0.020-0.060, and the rest is a small amount of uncontrollable impurity elements. According to the low carbon content of the steel grade, the solidification shrinkage of the low carbon steel is low, and the thickness of the shell in the crystallizer is also relatively thin. Based on this, in order to obtain a solidification shell with uniform thickness, the heat transfer in the crystallizer needs to be properly controlled. The designed protective slag should have a low crystallization temperature, so that the heat flow through the slag film is large enough to increase the thickness of the shell sensitive to sticking to avoid leakage.

[0004] Li2O is a strong fluxing agent, even if the Li2O content in the slag is low, it has a greater impact on the melting temperature. The trace addition of Li2O (Li2O < 2%) improves the glassification degree of the protective slag, and the crystallization rate is reduced, but the excessive addition of Li2O (Li2O greater than 4%) will greatly reduce the glassification degree due to the large amount of melilite crystals precipitated. Therefore, the appropriate addition of Li2O can obtain a low-melting-point, low-viscosity, and good glassy protective slag, and the appropriate amount is Li2O < 2%. Since Li2O is not stable in air, the fluxing agent added in actual slag production is lithium carbonate or spodumene. Lithium carbonate begins to decompose into Li2O (fluxing) and carbon dioxide (escaping, which has little effect on the properties of the slag) at the boiling point (618°C).

[0005] Currently, lithium resources are mainly concentrated in a few countries such as Chile, Bolivia, Argentina, Australia, the United States, and China, and mainly in lithium-containing salt lakes. With the breakthrough of lithium extraction technology from salt lakes, salt lakes have replaced ores as the main raw material source for lithium salt production. However, due to the remote geographical location and high altitude of lithium-containing salt lakes in China, the effective mining time is short, and the lithium-magnesium ratio of salt lakes is high, making separation difficult. So far, there has been no large-scale development, and the main method is to import spodumene for lithium extraction. Spodumene is relatively rare, resulting in high purchase prices. At the same time, China has abundant lepidolite resources, mainly in Yichun, Jiangxi, with proven lithium oxide reserves of 1.1 million tons, accounting for 31% of the country's ore reserves and 12% of the world's ore reserves. Lepidolite (KLi 2-x Al 1+x [Al 2x Si 4-2x O 10 ](F, OH)2}) is a stable continuous layered tetrahedral fluorine-containing aluminosilicate mineral belonging to the monoclinic layered structure, which is composed of aluminum-oxygen octahedra and silicon-oxygen tetrahedra, and Li + , K + , etc. fill the octahedral positions in the structure. Compared with spodumene, lepidolite has a complex mineral composition, low Li2O content, and contains 5.9% fluorine. To smelt industrial-grade lithium carbonate products, a series of complex processes are required, which generates a large amount of waste slag and consumes a large amount of energy. Therefore, rational development and utilization of China's lepidolite resources are of great significance to meet the domestic demand for lithium salts.

[0006] In recent years, the rise of new energy vehicle industry has led to a sharp rise in the prices of raw materials such as nickel, cobalt and lithium in battery materials. The unit price of industrial-grade lithium carbonate has reached nearly 500,000 yuan / ton, while the price of lepidolite is only 17,000 yuan / ton. Assuming that the final effective component of the protective slag contains 0.5% lithium oxide, the configuration of industrial-grade lithium carbonate requires 8,000 yuan / ton, while the configuration of the same target component protective slag with lepidolite only requires 4,000 yuan / ton.

[0007] From the literature search, there are some researches on the design and manufacturing method of austenitic stainless steel. The patent with the application publication number CN112605356A, “Austenitic stainless steel continuous casting crystallizer special protective slag and its application”, the protective slag is suitable for high-strength high-nitrogen (content around 0.25%) austenitic stainless steel continuous casting, which can solve the problem of QN1803 and GN1601 series austenitic stainless steel that cannot be produced smoothly and produces casting defects due to the lack of special protective slag. The viscosity of the protective slag is 0.35-0.65 Pa·s at 1300℃, which is too large to facilitate the smooth downward of the initial solidification shell; and the binary basicity is 0.6-0.75, which cannot provide enough crystallization layer to meet the heat control requirements of the stainless steel shell; in addition, the configuration of 0.5-2% lithium oxide with lithium carbonate will bring huge production pressure to the steel industry which already has low profit.

[0008] The patent with the application publication number CN110614351A, “Stainless steel continuous casting protective slag and its preparation method”, provides a kind of continuous casting protective slag for stainless steel, which makes the surface cleaning rate of the cast duplex stainless steel billet reach more than 98%, and the quality of the duplex stainless steel billet is good. But its binary basicity is 1.35-1.48, and a large amount of crystals are precipitated during use, which cannot conduct heat in time, so that the billet shell cannot be cooled in time and the thickness is too thin, which can easily lead to the problem of leakage. And the main application object is austenitic and ferritic duplex stainless steel, which still cannot avoid the denaturation of the protective slag caused by the absorption of Cr2O3 in the liquid slag during the continuous casting process of austenitic stainless steel, and further causes the problem of slag entrapment. SUMMARY

[0009] In order to overcome the shortcomings and problems of the existing austenitic stainless steel continuous casting process protective slag caused by the penetration of Cr2O3, which leads to the denaturation of the protective slag and the decline of product quality, and the high price of traditional slag-making material industrial-grade lithium carbonate and lithium spar, the purpose of the present application is to provide a new type of austenitic stainless steel continuous casting crystallizer protective slag and its application.

[0010] The present application aims at the technical problem of the slag entrapment of the 300 series austenitic stainless steel continuous casting process caused by the change of the protective slag in the crystallizer, by introducing appropriate lithium mica into the traditional austenitic stainless steel protective slag to replace the industrial lithium carbonate and the imported spodumene in the conventional slag material, and cooperating with other appropriate components, the energy consumption is reduced, the production cost of the protective slag is reduced, and the problem of the change of the protective slag caused by the Cr2O3 infiltration is solved. The present application reduces the frequency of the slag entrapment of the high speed stainless steel continuous casting process, significantly improves the surface quality of the casting blank, and brings good economic benefits to the enterprise. The present application is a new type of continuous casting crystallizer protective slag for austenitic stainless steel, which is composed of the following components in mass percentage:

[0011] Wollastonite 15.00-35.00%, lithium mica 15.0-25.00%, fluorite 13.00-19.00%, limestone 11.00-19.00%, sodium carbonate 8.00-12.00%, glass slag 3.50-6.50%, borax pentahydrate 3.00-5.00%, magnesia 1.00-3.00%, manganese carbonate 0.50-2.50%, graphite 0.50-2.00%, and carbon black 0.50-2.00%.

[0012] Preferably, the protective slag is composed of the following components in mass percentage:

[0013] Wollastonite 18.00-32.00%, lithium mica 17.00-23.00%, fluorite 14.00-18.00%, limestone 12.00-18.00%, sodium carbonate 8.50-11.50%, glass slag 4.00-6.00%, borax pentahydrate 3.50-4.50%, magnesia 1.50-2.50%, manganese carbonate 0.75-2.25%, graphite 0.7-1.75%, and carbon black 0.7-1.75%.

[0014] Further preferably, the protective slag is composed of the following components in mass percentage:

[0015] Wollastonite 21.00-29.00%, lithium mica 18.50-21.50%, fluorite 15.00-17.00%, limestone 13.00-17.00%, sodium carbonate 9.00-11.00%, glass slag 4.50-5.50%, borax pentahydrate 3.75-4.25%, magnesia 1.75-2.25%, manganese carbonate 1.00-2.00%, graphite 0.7-1.50%, and carbon black 0.7-1.50%.

[0016] More preferably, the protective slag is composed of the following components in mass percentage:

[0017] Wollastonite 24.5-25.5%, Lithium mica 19.50-20.5%, Fluorite 15.5-16.5%, Limestone 14.5-15.5%, Sodium carbonate 9.5-10.5%, Glass slag 4.75-4.85%, Borax pentahydrate 3.95-4.05%, Magnesite 1.95-2.05%, Manganese carbonate 1.45-1.55%, Graphite 0.8-0.9%, Carbon black 0.8-0.9%.

[0018] Still further preferably, the protective slag consists of the following components in mass percentage:

[0019] Wollastonite 25%, Lithium mica 20%, Fluorite 16%, Limestone 15%, Sodium carbonate 10%, Glass slag 4.8%, Borax pentahydrate 4.0%, Magnesite 2.0%, Manganese carbonate 1.5%, Graphite 0.85%, Carbon black 0.85%.

[0020] The designed protective slag has a starting melting temperature interval of 995-1095°C, a complete melting temperature interval of 1330-1390°C, and a melting speed of 34-45s. Preferably, the protective slag has a starting melting interval of 1010-1080°C, a complete melting interval of 1340-1380°C, and a melting speed of 37-42s.

[0021] Preferably, the protective slag has a kinematic viscosity of 0.08-0.20 Pa·s at 1300°C and a turning temperature of 1120-1220°C.

[0022] Preferably, the protective slag has a binary basicity of 1.00-1.10.

[0023] The application of a new type of protective slag for continuous casting mold for austenitic stainless steel, the protective slag is used as a protective slag for continuous casting mold for the process of continuous casting of austenitic stainless steel; the Cr content in the austenitic stainless steel is 17.50-19.00wt%, and the Ni content is 7.50-8.50wt%.

[0024] As a further preference, the application of a new type of protective slag for continuous casting mold for austenitic stainless steel, the austenitic stainless steel includes the following components in mass percentage: C: 0.025-0.065%; Si: 0.30-0.70%; Mn: 0.50-2.00%; P: ≤0.05%; S: ≤0.006%; Cr: 17.50-19.00; Ni: 7.50-8.50; Mo: ≤0.60%; Cu: 0.50-1.00; N: 0.020-0.060; and the balance is iron.

[0025] As a further preference, the application of a new type of protective slag for continuous casting mold for austenitic stainless steel, the protective slag has a slag consumption of 0.25-0.45kg / t.

[0026] In industrial applications, the lithium mica used in the present application can be a low lithium content lithium mica, such as a lithium mica with a Li2O content of less than or equal to 2.8wt% calculated as Li2O.

[0027] Compared with CN112605356A and the prior art, the present application first realizes that when the addition amount of Li2O is less than 0.6%, a defect-free 304 stainless steel white skin coil can also be obtained.

[0028] Compared with the prior art, the present application has the following advantages and positive effects:

[0029] 1. The new type of continuous casting mold protective slag for austenitic stainless steel disclosed by the present application, by adding lithium mica ore in the raw material instead of expensive industrial-grade lithium carbonate and lithium spar which mainly depends on import in the traditional slag-making material, not only reduces the production and procurement cost, but also shortens the smelting process, and also reduces the energy consumption caused thereby.

[0030] 2. With the cooperation of other appropriate components, the trace amount of Li2O can better inhibit the problem of slag entrapment caused by the denaturation of the protective slag due to the absorption of Cr2O3 in the liquid slag during the continuous casting process of austenitic stainless steel, reduce the incidence of line defects caused by slag entrapment, and significantly improve the surface quality of the cast slab and the subsequent hot-rolled coil, thereby bringing good economic benefits to the enterprise. It provides the necessary conditions for the true industrial application of the protective slag.

[0031] 3. The present application is simple and easy to implement, can be used for large-scale production of steel continuous casting process, and has high practical application value. Its investment in the production line will bring good social and economic benefits.

[0032] In summary, the present application aims at the technical problem of slag entrapment caused by the denaturation of the protective slag in the continuous casting process of austenitic stainless steel, by replacing the expensive industrial-grade lithium carbonate and the imported lithium spar in the traditional slag-making material with lithium mica in the raw material of the traditional austenitic stainless steel protective slag, and introducing an appropriate amount of Li2O to solve the problem of slag entrapment caused by the denaturation of the protective slag due to the penetration of Cr2O3. At the same time, the present application is simple and easy to implement, can be used for large-scale production of steel continuous casting process, and has high practical application value. Its investment in the production line will bring good social and economic benefits. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 The macroscopic morphology diagram of the 304 stainless steel white skin coil caused by the slag entrapment of the protective slag developed for Comparative Example 1,

[0034] Figure 2 The surface macroscopic morphology diagram of the 304 stainless steel white skin coil obtained when the protective slag developed for Example 3 is used for 304 stainless steel continuous casting. DETAILED DESCRIPTION

[0035] The application is further illustrated below with reference to Examples 1-3 and Comparative Examples 1-2. The examples are intended to illustrate the application only and are not intended to limit the application in any way. Table 1 lists the chemical composition of the protective slag of Examples 1-3 and Comparative Examples 1-2, Table 2 lists the physicochemical properties of the protective slag of Examples 1-3 and Comparative Examples 1-2, and Table 3 lists the data statistics of the quality inspection results of the white skin plates, Figure 1 Fig. 1 is a macroscopic morphology diagram of the white skin coil of 304 stainless steel caused by the protective slag, Figure 2 Fig. 2 is a macroscopic morphology diagram of the surface of the white skin coil of 304 stainless steel without defects (normal).

[0036] Examples 1-3 and Comparative Examples 1-2:

[0037] Preparation process:

[0038] 1) The industrial raw materials such as lithium mica, glass slag, limestone, wollastonite, sodium carbonate, fluorite, borax pentahydrate, magnesia, manganese carbonate, graphite, carbon black, etc. are weighed according to the protective slag component content listed in Table 1, with the error not exceeding 1%; then the weighed raw materials are mixed and mechanically stirred to make the components uniformly mixed;

[0039] 2) The mixed sample is poured into a graphite crucible and heated to melt in a medium-frequency induction furnace, and a certain period of time is kept for removing volatile components and uniformly melting the slag components;

[0040] 3) The molten liquid slag is poured into water for rapid cooling to obtain a glassy sample;

[0041] 4) The glassy sample is ground, and a required amount of carbonaceous material (graphite 0.85%, carbon black 0.85%) and carboxymethyl cellulose binder (2%) are added to prepare a slurry.

[0042] 5) The obtained slurry is sent into a spray granulation drying tower for drying and granulation, and the product is required to have a moisture content of less than 0.5% and a particle size in the range of 0.05-1.5 mm, and is sealed in a bag for later use.

[0043] Table 1 Chemical composition (wt%) of the protective slag of Examples 1-3 and Comparative Examples 1-2

[0044]

[0045] The prepared protective slag of Examples 1-3 and Comparative Examples 1-2 is tested for melting and viscosity properties by SHTT (single filament thermocouple technology) and high-temperature viscometer, and the obtained physicochemical property test results of the protective slag are shown in Table 2.

[0046] Table 2 Physicochemical property summary table of the protective slag of Examples 1-3 and Comparative Examples 1-2

[0047] Start melting temperature / °C Complete melting temperature / °C Melting speed / s Viscosity at 1300°C / Pa s Example 1 1031.6 1358.7 40 0.129 Example 2 1024.2 1353.5 39 0.113 Example 3 1013.5 1342.2 38 0.104 Comparative Example 1 1056.4 1386.1 43 0.147 Comparative Example 2 1048.7 1376.3 41 0.138

[0048] From Table 2, it can be seen from the performance tests of Comparative Examples 1-2 and Examples 1-3 that the starting melting temperature, complete melting temperature and melting rate of the protective slag in Examples 1-3 are lower than those of Comparative Examples 1-2, indicating that the introduction of an appropriate amount of lepidolite into the existing protective slag has the effect of reducing the melting point. The viscosity of Examples 1-3 at 1300℃ is 0.129 Pa·s, 0.113 Pa·s and 0.104 Pa·s, respectively, while that of Comparative Examples 1 and 2 is 0.147 Pa·s and 0.138 Pa·s, respectively, indicating that the addition of lepidolite to the protective slag indeed has the effect of reducing the viscosity.

[0049] Further, the production of an appropriate amount of protective slag was tested on site in the continuous casting workshop (steel grade: 304 stainless steel; continuous casting speed: 1.1 m / min, the molten steel pouring temperature was controlled in the range of 1490±5℃, and the slag consumption was measured to be 0.28-0.32 t / kg during the entire experiment), and the test results are shown in Table 3:

[0050] Table 3 Data statistics table of quality inspection results of stripped white skin plate

[0051]

[0052]

[0053] (Note: the length of each roll of white skin after stripping is 1000 meters)

[0054] From Table 3, in terms of the number of line defects (determined by the product grade of the later stage of the table inspection instrument), the total number of line defects of Examples 1-3 is 78, 55 and 33 times, respectively, and the total number of line defects of Comparative Examples 1 and 2 is 113 times and 142 times, respectively. Whether it is the number of severe line defects or the number of moderate line defects, the total number of line defects of Examples 1-3 is lower than that of Comparative Examples 1 and 2, and the total number of line defects of Example 3 is lower than that of Comparative Examples 1 and 2 and Examples 1 and 2. Based on the melting performance, viscosity performance and on-site test quality inspection results, we believe that Example 3 has the best effect.

[0055] Further combining the physical and chemical performance analysis of the examples and comparative examples, it can be seen that the melting temperature, melting rate and viscosity of examples 1-3 are lower than those of comparative example 1 and comparative example 2, which will be beneficial to the melting of the mold flux in the crystallizer at high pulling speed, forming enough liquid slag layer to improve the lubricating performance, and being beneficial to the lateral heat transfer of the initial solidification shell in the crystallizer, which can meet the performance requirements of the mold flux at high pulling speed. The melting temperature and viscosity of comparative example 2 are higher than those of examples 1-3 because only a small amount of lepidolite is added, which cannot meet the performance requirements of the mold flux at high pulling speed, and thus the use effect is the worst. This shows that the addition of lepidolite in the mold flux can effectively inhibit the change of the performance of the mold flux in the crystallizer due to the penetration of Cr2O3, which increases the melting point and viscosity, and further causes the adverse effects of the slag-coated billet on the surface quality of the product after the hot rolling plate is washed.

[0056] The above examples 1-3 and comparative examples 1-2 show that the method for designing a new type of mold flux for continuous casting crystallizer of austenitic stainless steel proposed by the present application is feasible, and has certain practical value and industrial application potential.

[0057] The above examples and comparative examples are only for illustrating the present application, but not limiting the present application. The researchers in the related technical field can freely make various changes and modifications without departing from the spirit and scope of the present application, and thus all equivalent technical solutions should belong to the scope of the present application, which should be limited by the claims.

Claims

1. A novel continuous casting mold powder for austenitic stainless steel, characterized by: The protective slag consists of the following components in mass percentage: Wollastonite 15.00~35.00%, lepidolite 15.0~25.00%, fluorite 13.00~19.00%, limestone 11.00~19.00%, sodium carbonate 8.00~12.00%, glass slag 3.50~6.50%, borax pentahydrate 3.00~5.00%, magnesia 1.00~3.00%, manganese carbonate 0.50~2.50%, graphite 0.50~2.00%, carbon black 0.50~2.00%.

2. A novel continuous casting mould powder for austenitic stainless steel according to claim 1, characterized in that: The protective slag consists of the following components in mass percentage: Wollastonite 18.00~32.00%, lepidolite 17.00~23.00%, fluorite 14.00~18.00%, limestone 12.00~18.00%, sodium carbonate 8.50~11.50%, glass slag 4.00~6.00%, borax pentahydrate 3.50~4.50%, magnesia 1.50~2.50%, manganese carbonate 0.75~2.25%, graphite 0.7~1.75%, carbon black 0.7~1.75%.

3. A novel continuous casting mould powder for austenitic stainless steel according to claim 1, characterized in that: Wollastonite 21.00~29.00%, lepidolite 18.50~21.50%, fluorite 15.00~17.00%, limestone 13.00~17.00%, sodium carbonate 9.00~11.00%, glass slag 4.50~5.50%, borax pentahydrate 3.75~4.25%, magnesia 1.75~2.25%, manganese carbonate 1.00~2.00%, graphite 0.7~1.50%, carbon black 0.7~1.50%.

4. A novel continuous casting mould powder for austenitic stainless steel according to claim 1, characterized in that: Wollastonite 24.5~25.5%, lepidolite 19.50~20.5%, fluorite 15.5~16.5%, limestone 14.5~15.5%, sodium carbonate 9.5~10.5%, glass slag 4.75~4.85%, borax pentahydrate 3.95~4.05%, magnesia 1.95~2.05%, manganese carbonate 1.45~1.55%, graphite 0.8~0.9%, carbon black 0.8~0.9%.

5. A novel continuous casting mould powder for austenitic stainless steel according to any one of claims 1 to 4, characterized in that: The protective slag has a binary basicity of 1.00~1.

10.

6. Use of a new type of continuous casting mould powder for austenitic stainless steel according to any one of claims 1 to 4, characterized in that: The protective slag is used as a continuous casting mold protective slag for an austenitic stainless steel continuous casting process; the austenitic stainless steel has a Cr content of 17.50~19.00wt% and a Ni content of 7.50~8.50wt%.

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